For such men are false apostles, deceitful workmen, disguising themselves as apostles of Christ. – 2 Corinthians 11:13
The Bible speaks often about false prophets who can lead followers astray. Back in Paul's day, apparently there were false apostles.
Yes, there was fraud back in the day as well. There always is. Someone will try to take advantage of your trust in order to fulfill their selfish gain. A thief uses identity theft to impersonate someone, usually in order to steal money. There's no motive for someone to take on the effort to impersonate someone otherwise, especially if being an apostle of Christ means suffering persecution.
That's why Paul always begins and ends his letters with his unique signature, which is a hint to his readers that he actually is Paul.
Or did I commit a sin in humbling myself so that you might be exalted, because I preached God's gospel to you free of charge? I robbed other churches by accepting support from them in order to serve you. – 2 Corinthians 11:7-8
Paul's sarcastic remark is funny. Maybe he should have charged to preach God's gospel. But he didn't because it's something he freely received. God doesn't charge for it either.
Other opportunists probably want in on the donated money involved. They might also write letters to the church of Corinth pretending to be apostles, and thus demanding the support payment.
And when I was with you and was in need, I did not burden anyone, for the brothers who came from Macedonia supplied my need. So I refrained and will refrain from burdening you in any way. – 2 Corinthians 11:9
Paul demonstrates his pure intentions by not relying on the church's ministry fund. He works with his own hands in order to earn his living expenses to prove that he is genuine.
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